The use of precious metal catalysts such as platinum, palladium and rhodium to convert NO.sub.x and CO in effluent gases has generated an industry focused on emission controls. Prior art emission control devices are typically configured to be inserted within the exhaust stream from internal combustion engines, turbines and steam boilers. Ceramic catalyst supports upon which the precious metals are deposited, are provided in bulk. The bulk catalyst support is used to introduce the active catalyst to the gaseous stream which flows through the catalyst support. Due to the varying temperature range experienced under engine operating conditions, there is cyclic expansion and contraction of the ceramic supports. This thermal cycling eventually leads to degradation of the ceramic support, which in turn reduces catalyst activity, and also leads to loose or broken ceramic pieces falling into the engine manifold, with obvious deleterious effects.
The present invention provides a modular approach to catalyst support. Rather than providing a single bulk catalyst, the instant device utilizes a series of catalyst support tiles which are individually structurally supported and which can be individually replaced. According to the present invention, the ceramic catalyst support tiles are provided in preassembled steel frames and the tile faces are encased in steel mesh. This modular approach provides for easy maintenance, increased performance, lower replacement cost, interchangeability and easy adjustments.
Catalytic converters need periodic cleaning to maintain catalyst activity and to minimize any back pressure on the engine resulting from reduced flow capacities in the converter device. The cleaning process consists of removing the ceramic catalyst support and immersing the catalyst support in an acid solution. The present invention, which provides individually contained and structurally sound catalyst support tiles, has significant advantages over bulk catalysts. Since the instant catalyst support tiles are protected by a wire mesh and steel frame, no special holding tools are required to remove the catalyst support for cleaning. Further, the containment system of the present invention permits reuse of ceramic catalyst support tiles which may be broken or cracked. Bulk catalyst supports, on the other hand, must be specially handled, and any broken catalyst support tiles must be replaced.
Individually framing catalyst support tiles also permits the use of a greater range of tile thicknesses. Observed experience demonstrates that most catalytic reactions occur on the catalyst support tile surface. Therefore, catalyst performance is improved by stacking several thinner catalyst support tiles over the same total thickness as would be used in bulk catalyst support systems. The use of steel frames for edge structural support permits use of generally thinner catalyst tiles, at greatly improved overall catalyst efficiency.
During routine maintenance, the present invention also provides for lower replacement cost. Experience has shown that the most permanent contamination is accumulated on the face of the first catalyst tile facing the inlet stream. After a certain period of use, the catalyst support tile has to be replaced due to clogged pores and reduced catalyst activity. The ability to use thinner catalyst support tiles permits replacement of the front catalyst support tile only, at reduced expense compared to the replacement of an unsupported thicker tile, as in the prior art devices.
Further, the modular approach of the instant invention permits interchangeability of the catalyst support tiles. The total quantity of catalyst required to effect conversion is proportional to the volume of exhaust flow from the emission source. Due to the modular approach of the present invention, individually supported tiles can be easily mixed and exchanged from one catalytic conversion unit to another. If it is desired to use the same catalytic converter for a higher flow engine or boiler, capacity can be easily increased by the addition of catalyst support tiles without sacrificing performance. Similarly, catalyst support tiles can be removed to provide for reduced capacities.